Failure Mechanism Analysis of Capacitor(I)

Effect of moisture on deterioration of Electrical parameters.

When the humidity in the air is too high, the water film is condensed on the surface of the capacitor shell, which can reduce the surface insulation resistance of the capacitor. Here, for a semi-sealed structure capacitor, moisture can also penetrate into the capacitor medium, so as to reduce the insulation resistance insulation ability of the capacitor dielectric. Therefore, the effects of high temperature and high humidity on the deterioration of capacitor parameters are very significant. The electrical properties of capacitors can be improved after drying and dehumidifying, but the consequence of water molecule electrolysis cannot be eradicated. For example, when capacitors work at high temperature, water molecules are electrolyzed to H + and OH-O + under the action of electric field, and electrochemical corrosion occurs at the root of the lead. Even when drying and dehumidifying, it is impossible to recover the lead wire.

2.2. Consequences of silver ion migration.

Most inorganic dielectric capacitors use silver electrodes. When semi-sealed capacitors work at high temperature, water molecules infiltrating into the capacitors produce electrolysis. The oxidation reaction occurs at the anode and the silver ion binds with the hydroxide ion to form silver hydroxide. A reduction reaction occurs at the cathode and silver hydroxide reacts with hydrogen ions to produce silver and water. As a result of the electrode reaction, the silver ions of the anode are continuously reduced to discontinuous metal silver particles to the cathode, and connected to the water film to form a tree extending to the anode. Silver ion migration not only occurs on the surface of inorganic medium, but also diffuses into the inorganic medium, resulting in the increase of leakage current and the complete short circuit between the two silver electrodes, which leads to the breakdown of the capacitor.

Silver ion migration can seriously destroy the silver layer on the surface of the positive electrode. Silver oxide with semiconductor properties is spaced between the lead solder joint and the silver layer on the electrode surface, which increases the equivalent series resistance of inorganic dielectric capacitor and increases the loss of metal part. The tangent of the loss angle of the capacitor increases significantly.

As the effective area of the positive electrode decreases, the capacitance of the capacitor will decrease as a result. The surface insulation resistance is reduced by the existence of silver oxide semiconductors on the dielectric surface between the two electrodes of inorganic dielectric capacitors. When the silver ion migration is serious, a dendritic silver bridge is built between the two electrodes, which greatly reduces the insulation resistance of the capacitor.

In conclusion, silver ion migration will not only worsen the electrical properties of unsealed inorganic dielectric capacitors, but also may lead to a decrease in dielectric breakdown field strength, and finally lead to capacitor breakdown.

It is worth mentioning that the silver electrode low-frequency ceramic monolithic capacitor is more serious than other types of ceramic dielectric capacitors because of the silver ion migration. The reason lies in the primary firing process and multilayer laminated structure of the capacitors. During the first sintering process of silver electrode and ceramic medium, silver participates in the solid state reaction of ceramic medium surface and infiltrates into the interface layer between porcelain and silver contact. If the ceramic medium is not dense enough, the silver ion migration can occur not only on the surface of the ceramic medium, but also through the ceramic dielectric layer after water infiltration. There are many gaps in multilayer laminated structure, the position of electrode is not accurate, the margin of dielectric surface is small, the silver paste permeates into the gap when the two ends of laminated layer are coated with external electrode, the insulation resistance of dielectric surface is reduced, and the path between electrodes is shortened. The phenomenon of short circuit is easy to occur when silver ions migrate.